The role of mesoscale structures in the disturbance of the stratosphere by two major events in 2020 and 2022

High-explosivity volcanic eruptions and extreme-intensity fires can inject pollutants into the upper troposphere and stratosphere (UTS), generating persistent disturbances of its composition and of the stratospheric aerosol layer, affecting the radiative balance and the climate system on a global scale.

We focus on two recent events, the largest known plume generated by a forest fire on 2020 new year in Australia (AF) with an amplitude comparable to a large volcanic eruption and the Hunga submarine eruption in January 2022, which has generated the largest stratospheric disturbance since the Pinatubo eruption. Its plume has been exceptional by its altitude reach 58 km and the massive injection of water vapour (10% instantaneous increase in the stratosphere).

In both cases, we take advantage of the large number and diversity of spaceborne instruments to analyze and revisit the properties of the stratospheric plumes, in particular the role of confinement in mesoscale structures dynamically induced from the radiative forcing. In the case of the AF, rising anticyclonic smoke vortices were formed by shortwave absorption and maintained compact for several months during which the confinement maintained the high concentration in aerosol and an anomalous chemical composition with a persisting moist air depleted in ozone. The ozone anomaly in the ozone column is also partly dynamical due to the fast rising motion of the vortices when they cruise in the high latitude summer stratosphere. In the case of the Hunga eruption, condensing water vapour was first essential to get rid of most of the ashes during the first hours following the eruption. The strong emission of the remaining water vapour in the stratosphere generated a pair of rapidly descending structures, also anticyclonic, that eventually broke after about a couple of weeks but were essential in maintaining a confinement able to convert SO2 to sulfates at an unprecedented rate. Such compact structures are in fact expected in any plume submitted to localized internal warming or cooling. The confinement process is discussed in relation with the various estimates of SO2 and sulfate, in particular a mesoscale resolving product with IASI which shows the fast conversion to sulfate in mesoscale structures. The longer-term impact is diagnosed with SAGE III, showing how the rapid growth led to larger than usual aerosols with a sharper distribution, differing from other eruptions and providing large radiative impact in spite of the relatively small amount of emitted SO2. In passing, these unusual characters are the main reason of the dispersion of measurements of limb scattered instruments for this event. We show on the contrary that the SAGE III measurements are in very good agreements with the CALIOP estimates of extinctions for the isolated plume.